1. Technical Field
The present invention relates to a method for manufacturing new vehicle lighting devices, and particularly to a technique, employed by a vehicle lighting device manufacturing method for joining a transparent cover and a housing using ray welding, whereby secure joining of the transparent cover and the housing is ensured.
2. Related Art
For the manufacture of vehicle lighting devices, such as automobile lighting devices, a process is required for attaching a transparent cover to a housing to cover the front opening of the housing.
Conventionally, for the attachment of the transparent cover to the housing, a method is generally employed whereby seal legs are projected at the external edges of the transparent cover, while a groove is formed in the housing to accept the seal legs, so that the seal legs of the transparent cover are fitted to the groove of the housing with a sealing material or an adhesive material. Using this attachment method, however, since the seal legs and the groove are located at a portion extending outward from the external edges of the transparent cover and the housing, a comparatively large light blocking portion, i.e., a dark portion, is formed at the external edge area when the transparent cover is viewed from the front. Thus, the light emitting area is small when compared with the overall size of the transparent cover.
To resolve this problem, a method for joining the seal legs of the transparent cover directly to the face of the housing has been proposed. With this method, when the seal legs of the transparent cover and the housing are joined directly, only a small joining margin is required, so that only an extremely narrow dark portion is formed at the outer edge of the transparent cover. Furthermore, hot plate welding or oscillating welding, for example, is generally employed as means for directly jointing the seal legs to the housing. According to this means used for the joining of the transparent cover and the housing, the materials of the transparent cover and the housing are melted and mixed at the joint and are then cooled and solidified. However, surplus molten material flows out of the joint and is visible from the front, through the transparent cover, and detracts from and adversely affects the external appearance.
Further, for various types of welding there are shape limitations (inclination angle limitations), e.g., for vibration welding, relative to the direction of vibration, a substantially flat state is required, and for hot plate welding, there can be no extreme inclination in the direction in which a hot plate is pressed. As a result, current three-dimensional design needs cannot be met.
Another method for joining a transparent cover and a housing is ray welding, such as laser welding. When laser welding is used, the molten state of the material, i.e., the result of energy irradiation at the joint, can be stabilized by controlling the output of the laser, the diameter of the laser spot at the joint and the speed at which scanning is performed. Therefore, the overflow of molten material, the result of excessive melting, can be resolved, and even a complicated shape can be handled. By using a laser beam to irradiate the housing through the transparent cover, the laser beam heats and melts the material of the housing while thermal conduction melts the material of the transparent cover, and the two materials are fused and pressure-bonded together.
According to the principles of laser welding, the welding portion of the transparent cover and the welding portion of the housing should contact each other precisely, otherwise a welding failure will occur.
However, since the transparent cover and the housing of a vehicle lighting device are resin molding products, it is actually difficult for products having shapes exactly as designed to be extracted, because shrinking, for example, can cause the products to be warped and otherwise malformed after molding. Accordingly, it is difficult for the transparent cover and the housing to precisely contact each other along the entire length of their external edges. And when the transparent cover and the housing do not precisely contact each other, heat applied to the housing by the laser will not be transmitted to the transparent cover, and the welding process will fail.
In order to eliminate the inferior contact condition that is the result of the molding process problem described above, and that adversely affects the welding of the portions of the two resin molding products that are to be joined using laser welding, a technique is disclosed in patent document 1. According to this technique, prior to the use of laser welding, the welding portions of two resin molding products are preliminarily heated to respective temperatures equal to or higher than the heat deformation temperatures of the resin materials of the two, and thus are softened. Then, when the two welding portions are actually pressure-bond to each other, the soft portions are deformed together, brought into close contact, and thereafter, the laser welding procedure is performed.
[Patent Document 1] JP-A-2004-74734